1. Technical Field
The present invention relates generally to stators for electric rotating machines that are used in, for example, motor vehicles as electric motors and electric generators, and more particularly to stators which include a stator core and a segment-type stator coil mounted on the stator core. Hereinafter, the term “a segment-type stator coil” denotes a stator coil which is comprised of a plurality of electric conductor segments.
2. Description of Related Art
There are known stators for electric rotating machines which include a segment-type stator coil for improving the space factors of the stator coil in slots of a stator core of the stator.
For example, Japanese Patent Application Publication No. 2008-199751 (to be simply referred to as Patent Document 1 hereinafter) discloses a stator which includes an annular stator core and a stator coil comprised of a plurality of substantially U-shaped electric conductor segments.
The stator core has a plurality of slots that are formed in the radially inner surface of the stator core and arranged at predetermined intervals in the circumferential direction of the stator core. Each of the slots extends in the axial direction of the stator core so as to penetrate the stator core in the axial direction.
Each of the electric conductor segments is substantially U-shaped to include a pair of straight portions that extend parallel to each other and a turn portion that connects ends of the straight portions on the same side.
In forming the stator coil, the straight portions are first axially inserted, from one axial side of the stator core, respectively into corresponding two of the slots of the stator core; the corresponding two slots are separated from each other by a predetermined pitch (e.g., a predetermined number of the slots).
Then, according to one embodiment of Patent Document 1, for each of the straight portions, a free end part of the straight portion, which protrudes outside of the corresponding slot on the other axial side of the stator core, is bent to form an oblique portion of the electric conductor segment. The oblique portion extends, along the circumferential direction of the stator core, obliquely at a predetermined angle with respect to the corresponding axial end face of the stator core. The oblique portion has a distal end part from which an insulating coat (or insulating film) that covers the electric conductor segment is removed; thus, the distal end part makes up an insulating coat-removed portion of the electric conductor segment.
Thereafter, each corresponding pair of the insulating coat-removed portions of the electric conductor segments, which are radially adjacent to each other, are joined, for example by welding, to form a joint therebetween. Consequently, the electric conductor segments are electrically connected to one another, thereby forming the segment-type stator coil.
With the above formation of the stator coil, all the oblique portions of the electric conductor segments together make up a coil end of the stator coil on the other axial side of the stator core. Further, to reduce the protruding height of the coil end (i.e., the oblique portions) from the corresponding axial end face of the stator core, it is necessary to reduce the predetermined angle between the oblique portions of the electric conductor segments and the corresponding axial end face of the stator core. However, with reduction in the predetermined angle, the distance between each circumferentially-adjacent pair of the joints formed between the insulating coat-removed portions of the electric conductor segments is accordingly reduced, thereby making it difficult to ensure electrical insulation between the electric conductor segments.
Moreover, with reduction in the predetermined angle, for each of the joints formed between the insulating coat-removed portions of the electric conductor segments, the distances from the joint to those oblique portions of the electric conductor segments which are circumferentially adjacent to the joint are also accordingly reduced. Therefore, the insulating coats that cover those oblique portions of the electric conductor segments may be thermally degraded by the heat input for the formation of the joint by welding. Consequently, the thermal degradation of the insulating coats may also make it difficult to ensure electrical insulation between the electric conductor segments.
On the other hand, according to another embodiment of Patent Document 1, for each of the straight portions, a free end part of the straight portion, which protrudes outside of the corresponding slot on the other axial side of the stator core, is bent to form an oblique portion, an axially-extending end portion and a bent portion of the electric conductor segment. The oblique portion extends, along the circumferential direction of the stator core, obliquely at a predetermined angle with respect to the corresponding axial end face of the stator core. The axially-extending end portion is formed at the distal end of the free end part of the straight portion so as to extend in the axial direction of the stator core (i.e., perpendicular to the corresponding axial end face of the stator core). The bent portion is formed in the shape of a circular arc between the oblique portion and the axially-extending end portion. Further, from the axially-extending end portion, the insulating coat that covers the electric conductor segment is removed; thus, the axially-extending end portion makes up an insulating coat-removed portion of the electric conductor segment. Furthermore, each corresponding pair of the insulating coat-removed portions of the electric conductor segments, which are radially adjacent to each other, are joined, for example by welding, to form a joint therebetween. Consequently, the electric conductor segments are electrically connected to one another, thereby forming the segment-type stator coil.
However, with the above formation of the stator coil, for each of the electric conductor segments, the cross-sectional shape of the electric conductor segment at the bent portions thereof is deformed into a trapezoidal shape. Therefore, to prevent interference between those of the electric conductor segments which are aligned with each other in a radial direction of the stator core, it is necessary to provide additional radial spaces between those electric conductor segments. Consequently, it may be difficult to minimize the size of the stator.
Moreover, each pair of the electric conductor segments to be joined together are arranged so as to radially overlap each other both at the corresponding axially-extending end portions and at the corresponding bent portions thereof. Therefore, in joining the pair of the electric conductor segments, it is necessary to move, using a special jig, the corresponding insulating coat-removed portions (i.e., the corresponding axially-extending end portions) of the electric conductor segments toward each other by a distance equal to the thickness of the insulating coats of the electric conductor segments. Accordingly, it is necessary to secure an additional space in the stator for arranging the special jig, thereby making it difficult to minimize the size of the stator.
U.S. Pat. No. 7,759,835 B2 (to be simply referred to as Patent Document 2 hereinafter) discloses a stator which includes a stator core having a plurality of slots formed therein and a stator coil comprised of a plurality of substantially S-shaped electric conductor segments mounted on the stator core.
Specifically, according to the disclosure of Patent Document 2, before being inserted into a corresponding one of the slots of the stator core, each of the electric conductor segments is first bent into an L-shape, resulting in a long leg and a first short leg. Then, the long leg of the L-shaped electric conductor segment is inserted into the corresponding slot of the stator core. After the insertion, the electric conductor segment is further bent into a substantially S-shape, resulting in a second short leg while reducing the length of the long leg. The first short leg of each one of a majority of the electric conductor segments is then welded to the first short leg of another one of the electric conductor segments on one axial side of the stator core. The second short leg of each one of a majority of the electric conductor segments is then welded to the second short leg of another one of the electric conductor segments on the other axial side of the stator core. As a result, the segment-type stator coil is obtained.
However, with the above formation of the stator coil, it is necessary to perform the task of welding the electric conductor segments on both axial sides of the stator core, thereby complicating the manufacturing process of the stator.
Further, to reduce the protruding heights of the first and second short legs of the electric conductor segments from the corresponding axial end faces of the stator core, it is necessary to reduce the angles between the first and second short legs of the electric conductor segments and the corresponding axial end faces of the stator core. However, with reduction in the angles, the distance between each circumferentially-adjacent pair of the welds formed between the electric conductor segments is accordingly reduced, thereby making it difficult to ensure electrical insulation between the electric conductor segments.
Moreover, with reduction in the angles, for each of the welds formed between the electric conductor segments, the distances from the weld to those first or second short legs of the electric conductor segments which are circumferentially adjacent to the weld are also reduced. Therefore, insulating coats that cover those first or second short legs of the electric conductor segments may be thermally degraded by the heat input for the formation of the weld. Consequently, the thermal degradation of the insulating coats may also make it difficult to ensure electrical insulation between the electric conductor segments.